132 CEYLON PEARL OYSTER REPORT. 



Genital Apparatus. The testes, which are very numerous and of large size, lie 

 between the gut branches at almost the same level as the ovaries, being only a triHe 

 more ventral in position. 



Terminal Ducts (see Plate, fig. 7). 



Male. The two vasa deferentia (v.d.) enter the anterior end of a small vesicula 

 seminalis (v.s.) which lies close against the dorsal body- wall. It is lined with a 

 ciliated epithelium, outside which is a narrow layer of circular muscle fibres ; it 

 tapers at its posterior distal end into a duct (c/.e.) without muscular walls, but with 

 rather a wide lumen which runs to the penis (pe.). At the base of the penis it joins 

 another duct which runs a short course directly from the penis to the prostate 

 gland (pr.), an elongate oval body of about twice the size of the vesicula seminalis, 

 lined with a well-developed secretory tissue enclosed in a layer of circular muscle 

 fibres. The prostate duct is very short. The antrum masculinum (a.m.) is of 

 moderate size, and in one of the specimens from which I prepared sections it is full of 

 the secretion from the prostate. 



Female Ducts. The vagina (va.) runs upwards from the female aperture, then 

 close to the dorsal body-wall it turns forward for a short distance and ends receiving 

 the two uteri. The shell-glands (sh.gl.) lie about the first part of its course. 



Thalamoplana, n. gen. 



Closely allied to Discocelis. Male and female apertures separated. Antrum 

 masculinum very large ; its walls carry muscular projections, at the free ends of which 

 lie the prostatic glands. The penis is of large size, truncate, and also carries prostatic 

 glands at its end. In other respects the genus is similar to Discocelis. 



Thalamoplana herdmaui, n. sp. Plate, figs. 2-5 and 8. 



Two specimens were found in the lagoon at Galle. 



Measurements. Length, about 25 millims. ; breadth, 17"5 millims. Buccal opening, 

 12 millims. from anterior end; male opening, 2 millims. behind buccal; female 

 opening, close behind, but quite distinct from male. 



Coloration. Dorsal surface white with brown mottling. Along the mid-dorsal 

 line is a brown frond-like pattern consisting of a median stripe bearing lateral lobes 

 or segments in pairs, which are irregular in shajie and size, but, roughly speaking, 

 smaller and larger pairs alternate. The ventral surface is white. 



Eye-si^ots. These are arranged much as in Discocelis tigrina (see fig. 3 on Plate). 

 On either side of the brain is a small elongate cluster of spots ; these clusters diverge 

 from each other posteriorly. Between them lie several diffusely scattered spots over 

 the brain. Marginal eyes are also present on the anterior margin. 



Body-wall. The epidermis consists of columnar ciliated cells which are of 

 considerably greater length on the ventral side of the body than on the dorsal. 

 There are no rhabdites, but the cells appear to contain secretions of the nature of 



